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A256934
Numbers k such that R_(k+2) + 8*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
0, 1, 12, 16, 33, 37, 42, 6643, 35157, 63202, 125292, 200746
OFFSET
1,3
COMMENTS
Also, numbers k such that (172*10^k - 1)/9 is prime.
Terms from Kamada.
a(13) > 250000.
EXAMPLE
For k=1, R_3 + 8*10^1 = 111 + 80 = 191 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(172*10^#-1)/9 ] &]
PROG
(Magma) [n: n in [0..300] | IsPrime((172*10^n-1) div 9)]; // Vincenzo Librandi, Apr 14 2015
(PARI) for(n=0, 300, if(isprime((172*10^n-1)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015
CROSSREFS
Cf. A002275.
Sequence in context: A020740 A344021 A086980 * A343153 A360619 A058203
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 13 2015
STATUS
approved